How it will be useful & when we will use this system & why most of the people use production system not quality??
Question is quite vague
Specific system are used as business requires that functionality
They could be 2 tier Dev - Prod or 3 Tier landscape Dev -QA - prod
Dev = Development
Q for replicating issues from prod and doing Final testing before sending to Prod
Prod for users to carry out the work in a live scenario
You can find more answers on SDN if you look for 2 tier vs 3 tier